Calculate Pregnancy Due Date From Last Period
You just found out you are pregnant. The first question: when is the baby due? Your last period date holds the answer.
Quick Answer
Count 280 days (40 weeks) from the first day of your last menstrual period (LMP). Use the free calculator at dotsapps.com to see your due date, current week, and trimester milestones.
How Due Dates Are Calculated
Doctors estimate your due date using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP).
This assumes a 28-day cycle and that you ovulated on day 14. If your cycle is longer or shorter, the due date may shift slightly.
Only about 5% of babies are born on their exact due date. Most arrive within two weeks before or after. It is an estimate, not a guarantee.
What If I Do Not Know My Last Period Date
If you are unsure of your LMP, an ultrasound can estimate your due date. First-trimester ultrasounds (before 13 weeks) are the most accurate, usually within 5-7 days.
If you know when you conceived, add 266 days (38 weeks) to that date. Conception typically happens about 14 days after the start of your last period.
Talk to your doctor if you are uncertain. They can help pin down a more accurate date.
Pregnancy Timeline: Trimesters and Milestones
Pregnancy is divided into three trimesters:
- First trimester (weeks 1-12) — Morning sickness, fatigue. Baby's organs begin forming.
- Second trimester (weeks 13-26) — Energy returns. You may feel the baby move around week 18-20.
- Third trimester (weeks 27-40) — Baby grows rapidly. You prepare for delivery.
Key milestones: heartbeat at week 6, gender visible at week 18-20, viability outside the womb around week 24, full term at week 37.
Early vs Late: When Babies Actually Arrive
The due date is week 40, but healthy babies arrive anywhere from week 37 to week 42:
- Early term: 37-38 weeks
- Full term: 39-40 weeks
- Late term: 41 weeks
- Post-term: 42+ weeks
First-time mothers tend to deliver a few days late. Second or later babies often come a bit earlier. Every pregnancy is different.
How to Do It: Step-by-Step
- 1
Open the Pregnancy Calculator at dotsapps.com
- 2
Enter the first day of your last menstrual period
- 3
See your estimated due date and how many weeks along you are
- 4
View trimester breakdown and upcoming milestones
- 5
Save the date and share with your partner or doctor
Frequently Asked Questions
How accurate is a due date calculator?
It is a good estimate. Only about 5% of babies arrive on the exact due date. Most come within two weeks before or after. An early ultrasound can improve accuracy.
Does due date change with irregular periods?
Yes. The standard calculation assumes a 28-day cycle. If your cycle is longer, your due date may be later. An ultrasound gives a better estimate for irregular cycles.
How many weeks is full term?
Full term is 39 to 40 weeks. Babies born at 37-38 weeks are early term and usually healthy. Before 37 weeks is premature.
Can my due date change during pregnancy?
Yes. If an ultrasound measurement differs significantly from the LMP estimate (by more than a week in the first trimester), your doctor may adjust the due date.
Is pregnancy really 9 months or 10?
Technically closer to 10 lunar months (40 weeks). But since calendar months vary in length, people round down to 9 months. It is 280 days from your last period.
Ready to Try It?
Pregnancy Due Date Calculator is free, private, and works right in your browser. No sign-up needed.
Open Pregnancy Due Date Calculator